Finding Instantaneous acceleration from a velocity-time graph

I know that the instantaneous acceleration is the slope of the tangent of that point and yes I do know I have to convert it to (m/s^2). But I am still not getting the right answers. Here are a list of answers I imputed and they are all wrong:
-.1
0
-2
-1
-1.9
1.9
-.278
-.52

on the first few, I forgot to convert. I used 58/31 the last time.....

The question had parts to it, but I don't know if they are necessary to solve this problem...here it is anyways:
A) Compute the average acceleration during the time interval t = 0s to t = 10s. 1.7
B) Compute the average acceleration during the time interval t = 30s to t = 40s. -1.7
C) Compute the average acceleration during the time interval t = 10s to t = 30s. 0
What is the instantaneous acceleration at t1 = 29 s. 0
I already computed this and got them right.